Vegetables are the edible leaves, roots, flowers, seeds, stems, and other parts of certain plants. The term “vegetable” is also used herein as the context requires to refer to the entire plant. Most vegetables are annuals in that they live for only one growing season.
When vegetables are grown commercially, it is common to fumigate the soil immediately before planting with a gas that kills weeds and organisms (e.g., nematodes). Methyl bromide is the most widely used fumigant, although it has been identified as an ozone depleting compound and its use is being curtailed under the provisions of the Montreal Protocol effective Jan. 1, 2005. Other common fumigants include chloropicrin, methyl chloride, ethylene dibromide, hydrogen cyanide, phosphine, carbon dioxide, methyl sodium, 1,3-dichloropropene, 3,5-dimethyltetrahydro-1,3,5-thiadiazin-2-thione, and sodium tetrathiocarbonate. To maximize the effectiveness of the fumigation and to reduce the escape of the fumigants into the atmosphere, the treated soil is covered with a film that slows the escape of the fumigant. These films are commonly known as fumigation films or barrier films and are typically removed after a few days.
During the growing season, it is common to cover the soil around the vegetables with a mulch. Mulch is a material that inhibits weeds, reduces wash-outs from heavy rains, traps moisture, and controls temperature by reflecting and/or absorbing sunlight. A variety of organic materials (e.g., wood chips, bark, pine needles, leaves, straw, hay) and inorganic materials (e.g., rocks and gravel) have been used as mulches. In recent years, the use of plastic films as mulch has increased. These films are commonly known as mulch films and are typically left in the field for the entire growing season. During the growing season, the films are contacted from below by the roots and stems of plants, rocks, soil particles, and the like, and are contacted from above by workers and machinery. The plastic mulch films are also subjected to prolonged exposure to sunlight.
A variety of plastic films (also known as sheets or sheetings) have been disclosed for use as mulches. Polyolefins (polymers of olefin units such as ethylene and propylene) have been used as mulches. Polyolefins are relatively inexpensive and have desirable physical properties, including tensile strength, impact and puncture resistance, tear resistance, flexibility, and resistance to damage from sunlight. Unfortunately, polyolefins are relatively porous. In other words, they have relatively high gas permeabilities (also known as diffusivities) so that the fumigants are able to pass through them relatively quickly. Therefore, they are poor barriers for fumigants.
Plastics that are effective fumigant barriers are generally stiff and have poor tear and puncture resistance properties. Such plastics include polyamides (an example of which is nylon) which contains repeating units of amides [—CH2—CHCONH—], ethylene vinyl alcohol copolymers (commonly known as EVOH) which contain repeating units of ethylene [—CH2—CH2—] and of vinyl alcohol [—CH2—CHOH—], polyethylene terephthalate (commonly known as PET) which contains repeating units of [—C10H8O4—], polyvinylidene chloride (commonly known as PVDC) which contains repeating units of vinylidene chloride [—CH2—CCl2—], and polyvinyl alcohol (commonly known as PVOH) which contains repeating units of vinyl alcohol [—CH2—CHOH—].
Various attempts have been made to provide multilayer plastic films for use as barriers and/or mulches. For example, Shoshani, Australian Patent Application No. 11107/92, published Aug. 27, 1992, discloses a coextruded multilayer plastic film for a barrier film. The film comprises a barrier layer such as polyamide, EVOH, PET, or polyurethane sandwiched between layers of polyolefin with optional adhesive tie layers. Multilayer plastic mulch films are also discussed in “Reducing Methyl Bromide Field Application Rates With Plastic Mulch Technology” by J. W. Nowling, The Institute of Food and Agricultural Services, University of Florida, published January 2002 and revised November 2005. There is no mention of sunlight reflecting/absorbing agents being present in these films.
As another example, Linde et al., U.S. Pat. No. 4,812,342, Mar. 14, 1989, disclose a tubular container consisting of an upper film sealed to a lower film at its edges. The lower film comprises a gas-permeable plastic such as polyolefin. The upper film comprises a coextruded multilayer barrier film comprising layers of a polyolefin, an adhesive, a barrier layer, another adhesive layer, and another layer of polyolefin. Suitable barrier layers for the upper film are saponified ethylene/vinyl acetate copolymers, polyvinylidene chloride, polyester, polyvinyl alcohol, and polyamide. The container is placed on the ground and a fumigant gas is added through a valve into the interior of the container. The container is held in place for a few days to allow the fumigant gas to diffuse through the lower film to the space above the soil. The container is then removed and, if desired, a separate mulch is applied to the field. The Linde et al. container is unsuitable for conventional fumigation because the lower film is so permeable that it would allow excessive amounts of the fumigant into the container.
As a further example, Basset et al., U.S. Pat. No. 5,846,661, Dec. 8, 1998, disclose, inter alia, a coextruded multilayer barrier film comprising layers of polyolefin, a coextrusion binder, a barrier layer, another coextrusion binder, and another layer of polyolefin. The barrier layer comprises a mixture of polyamide and polyolefin that may be plasticized and may optionally contain fillers such as carbon black. Unfortunately, the presence of blended polyolefin in the barrier layer increases its permeability and making the polyamide compatible with the polyolefin greatly increases the cost.
Accordingly, a demand exists for an improved method of growing vegetables. More particularly, a demand exists for a method of growing vegetables using a single multilayer barrier mulch film that is left in the field for fumigation and for the entire growing season.